IMPLEMENTATION MODULE StdIO;

CONST MaxStack = 32;

PROCEDURE m2writechar (c : CHAR);
  EXTERNAL;
PROCEDURE m2readchar () : CHAR;
  EXTERNAL;

VAR
  outStack : ARRAY [0 .. MaxStack - 1] OF ProcWrite;
  outTop   : INTEGER;
  inStack  : ARRAY [0 .. MaxStack - 1] OF ProcRead;
  inTop    : INTEGER;
  curOut   : ProcWrite;
  curIn    : ProcRead;

PROCEDURE DefaultWrite (c : CHAR);
BEGIN
  m2writechar(c)
END DefaultWrite;

PROCEDURE DefaultRead (VAR c : CHAR);
BEGIN
  c := m2readchar()
END DefaultRead;

PROCEDURE Write (ch : CHAR);
BEGIN
  curOut(ch)
END Write;

PROCEDURE Read (VAR ch : CHAR);
BEGIN
  curIn(ch)
END Read;

PROCEDURE PushOutput (p : ProcWrite);
BEGIN
  IF outTop < MaxStack THEN
    outStack[outTop] := curOut;
    INC(outTop)
  END;
  curOut := p
END PushOutput;

PROCEDURE PopOutput;
BEGIN
  IF outTop > 0 THEN
    DEC(outTop);
    curOut := outStack[outTop]
  END
END PopOutput;

PROCEDURE GetCurrentOutput () : ProcWrite;
BEGIN
  RETURN curOut
END GetCurrentOutput;

PROCEDURE PushInput (p : ProcRead);
BEGIN
  IF inTop < MaxStack THEN
    inStack[inTop] := curIn;
    INC(inTop)
  END;
  curIn := p
END PushInput;

PROCEDURE PopInput;
BEGIN
  IF inTop > 0 THEN
    DEC(inTop);
    curIn := inStack[inTop]
  END
END PopInput;

PROCEDURE GetCurrentInput () : ProcRead;
BEGIN
  RETURN curIn
END GetCurrentInput;

BEGIN
  outTop := 0; inTop := 0;
  curOut := DefaultWrite;
  curIn := DefaultRead
END StdIO.